In a conventional mold and injection molding process, a molten resin is injected into and fills a single mold cavity through a plurality of gates such that a plurality of resin streams are formed within the mold cavity. A weld forms at the position where the individual resin streams are united and fused. Even if a molten resin is filled into the mold cavity through a single gate, the resin may be separated into separate streams by a core or the like provided within the cavity such that again a weld forms at the position where the separated resin streams are united into a single stream. Further, if a molded article contains a portion having an uneven thickness, the molten resin injected and filled into the mold cavity is separated into separate streams due to the difference which occurs in the filing rate of the resin between the portion having a larger thickness and the portion having a smaller thickness, such that again a weld often forms at the portion where the separated resin streams are united into a single stream. In many cases, a weld in the molded article is formed due to a combination of the above-described factors. Accordingly, it is very difficult to produce a useful molded article having a complicated shape that is free from a weld. Unfortunately, the presence of a weld during molding poses very serious problems including not only the formation of a linear pattern called a weld mark which deteriorates the molded article appearance, but also the strength of the formed molded article is much lower than the inherent strength of the resin, because the molten resin streams at the weld are merely united and fused with each other and are not homogeneously mixed.
In order to solve the problem of low weld strength, Japanese Patent Laid-Open No. 71459/1973 discloses a molding process which comprises filling a resin into a mold which is provided with a resin well at a position where the weld occurs and pushing back the resin contained in the resin well into the mold, thereby improving the strength of the weld. Japanese Patent Laid-Open No. 202414/1990 discloses a molding process which comprises injecting and filling a resin into a mold cavity through a plurality of gates provided with a valve to form resin streams, uniting the resin streams and then operating the valve to give rise to a difference in the resin filling pressure to disturb the weld, thereby improving the strength of the weld.
In each of the above processes, however, it is difficult to disturb the orientation of a sufficient amount of resin or filler at the weld such that no satisfactory improvement in weld strength has been attained.